The optical scanning device that scans light by causing a mirror to oscillate is widely used in a digital copying machine, a laser printer, a barcode reader, a scanner, or a projector. With the recent development of a microfabrication technology, an optical scanning device, which illustrates the aforementioned device, and to which MicroElectro Mechanical Systems (MEMS) technology of is applied, has become a focus of attention.
The optical scanning device based on MEMS technology, in which a mirror mechanism can be integrally formed in a semiconductor process, is accordingly advantageous in that the device can be miniaturized. However, when a mirror size (chip size) increases, a cost problem occurs. Specifically, one wafer is expensive and, in view of yield or the number of chips actually produced therefrom, it is difficult to manufacture MEMS chips of 1 cm or more at low cost. Thus, the optical scanning device is not so suitable for use in high resolution projectors where a sufficiently large mirror size is required.
On the other hand, the optical scanning device based on MEMS technology has the following advantages. In such an optical scanning device, the mirror having both ends supported by beams that are made of an elastic member oscillates about an oscillation axis along the beams by a driving force such as an electrostatic force or an electromagnetic force, and accordingly optical scanning is carried out. Thus, unlike an optical scanning device of a type in which a polygon mirror or a galvano-mirror is rotated by a motor, a mechanical driving mechanism such as a motor is not necessary. As a result, the structure is simpler and assembling performance is higher, thereby contributing to lower costs. As compared with the aforementioned optical scanning device, the oscillation angle of the mirror can be set relatively large. This is important for realizing a projector that is capable of displaying an image on a large screen.
Thus, there is a demand for image display devices, such as projectors, that feature a large screen display and high resolution, that can be realized by applying the aforementioned advantages of the optical scanning device based on the MEMS technology to an optical device that has a large size mirror.
In the optical scanning device based on the MEMS technology, in many cases, to increase the oscillation angle of the mirror, a resonant mirror driven by the resonance frequency of the structure is used. The resonance frequency fr of the mirror is given byfr=(2π)−1(k/Im)1/2  (1)where k is a torsion spring constant of a torsion beam for supporting the mirror, and Im is the moment of inertia of the mirror.
When a driving force applied to the mirror is represented by T, the oscillation angle θ of the mirror is given byθ=QT/k  (2)where Q is a quality factor of the system, and typical values in air and vacuum are respectively about 100 and 1000.
In the resonant mirror, in many cases, high-speed vibration of about several 10 kHz as a resonance frequency fr is required. Accordingly, as the torsion beam, a torsion beam having a large torsion spring constant k, i.e., a hard torsion beam, is used (see Equation (1)). In such a case, the torsion spring constant k cancels out the quality factor Q of the mirror. As a result, oscillation angle θ of the mirror given by Equation (2) depends on driving force T. Thus, a large driving force is necessary to increase oscillation angle θ of the mirror.
On the other hand, in a certain type of optical scanning device of a certain type, a mirror is configured to be non-resonantly driven (i.e., DC-driven). In the case of this non-resonant mirror, oscillation angle θ of the mirror is given byθ=T/k  (3)where T and k are respectively a driving force applied to the mirror and torsion spring constant.
According to Equation (3), oscillation angle θ can be increased to some extent by increasing driving force T or decreasing torsion spring constant k. However, when torsion spring constant k is decreased, resonance frequency fr is decreased according to Equation (1). In such a case, since the resonance frequency approaches a driving frequency (normally, 60 Hz) in a non-resonant mode, a resonant waveform is superimposed on the driving waveform of the mirror. To prevent this, resonance frequency fr must be set to about 1 kHz or higher. As a result, torsion spring constant k cannot be greatly reduced. Thus, in the non-resonant mirror, as in the case of the resonant mirror, to increase oscillation angle θ of the mirror, the driving force must be greatly increased.
As described above, irrespective of the resonant type or the non-resonant type, to achieve a large oscillation angle of the mirror in the optical scanning device, it is important to secure a large driving force. In this regard, the use of a magnetic-force type driving device that generates a driving force with the aid of a permanent magnet and a coil is advantageous. The magnetic-force type driving devices are largely classified into the following two types depending on the arrangement of the permanent magnet and the coil.
(1) Movable Coil (MC) Type
For example, each of Patent Literatures 1 and 2 discloses a MC type driving coil in which a coil is mounted on a movable portion. A plurality of permanent magnets is disposed around the movable portion, and the movable portion is driven by Lorentz force applied on the coil when current is supplied to the coil.
(2) Movable Magnet (MM) Type
In a MM type driving device, a configuration where at least one permanent magnet is mounted on the plate surface of a platelike movable portion is frequently used. A coil is disposed near the movable portion, and the movable portion is driven by the magnetic interaction of the permanent magnet and the coil when current is supplied to the coil.
In such a driving device, various ideas for efficiently generating driving forces have been offered.
For example, Patent Literature 3 discloses a driving device including a coil disposed at a position facing the surface of a movable plate (movable portion) where a permanent magnet is mounted to incline according to the oscillation angle of the movable plate. Thus, even when the movable plate is deflected, a sufficiently large magnetic field is applied to the permanent magnet.
Patent Literature 4 discloses an optical scanner that includes a permanent magnet disposed on the rear surface of a mirror plate (movable portion) so that the magnetization direction can be horizontal, and a fixed yoke that houses a coil. The permanent magnet is held between the ends of the fixed yoke. Accordingly, a magnetic field applied to the permanent magnet through the fixed yoke can be relatively increased.